Electrochemical detection is attractive because it provides high sensitivity, small dimensions, low cost, fast response, and compatibility with microfabrication technologies, e.g. Hughes et al, Science, 254: 74-80 (1991); Mir et al, Electrophoresis, 30: 3386-3397 (2009); Trojanowicz, Anal. Chim. Acta, 653: 36-58 (2009); Xu et al, Talanta, 80: 8-18 (2009); and the like. These characteristics have led to the development of a variety of sensors based on amperometric, potentiometric or impedimetric signals and their assembly into arrays for chemical, biochemical and cellular applications, e.g. Yeow et al, Sensors and Actuators B 44: 434-440 (1997); Martinoia et al, Biosensors & Bioelectronics, 16: 1043-1050 (2001); Hammond et al, IEEE Sensors J., 4: 706-712 (2004); Milgrew et al, Sensors and Actuators B 103: 37-42 (2004); Milgrew et al, Sensors and Actuators B, 111-112: 347-353 (2005); Hizawa et al, Sensors and Actuators B, 117: 509-515 (2006); Heer et al. Biosensors and Bioelectronics, 22: 2546-2553 (2007); Barbaro et al, Sensors and Actuators B, 118: 41-46 (2006); Anderson et al, Sensors and Actuators B, 129: 79-86 (2008); Rothberg et al, U.S. patent publication 2009/0127589; Rothberg et al, U.K. patent application GB24611127; and the like. In particular, several of these developments involve the use of large-scale arrays of electrochemical sensors for monitoring multiple reaction steps on a large plurality of analytes confined to such an array, e.g. Anderson et al (cited above); Rothberg et al (cited above); and the like. Typically in such systems, analytes are randomly distributed among an array of confinement regions, such as microwells or reaction chambers, and reagents are delivered to such regions by a fluidics system that directs flows of reagents through a flow cell containing the sensor array. Microwells in which reactions take place, as well as empty wells where no reactions take place, may be monitored by one or more electronic sensors associated with each of the microwells.
Such systems are subject to a host of interrelated phenomena that make highly sensitive measurements challenging, particularly under low signal conditions. Such phenomena include unstable reference voltage for the electrical sensors, lack of knowledge as to which confinement regions contain analytes, variability in the amount of reagents delivered by a flow stream to analytes confined to different regions of an array potential mixing of successively delivered reagents, changes in instrument temperature, fluid leaks that may affect fluid potential, extraneous electrical interference, e.g. 60 Hz noise, cell phones, or the like, all of which may affect the quality of signals collected. In addition, for specific applications, there may further challenges related to particular reagents used, the sensitivity of a sensor for the analyte being measure, the presence or absence of interfering compounds, such as other reaction byproducts, and the like.
In view of the above, it would be advantageous to have available a system for carrying out multi-reagent electrochemical reactions in parallel on a large number of analytes which overcame the deficiencies of current approaches.